1. Field of the Invention
This invention relates to the field of data processing systems. More particularly, this invention relates to diagnostic mechanisms within data processing systems which are dynamically switchable between an operational mode and a powered down mode.
2. Description of the Prior Art
It is known to provide data processing systems with diagnostic mechanisms which can be used to perform diagnostic operations (e.g. software and hardware fault identification and analysis (debug)) upon the data processing systems so as to assist in the development of hardware, operating systems, application programs, overall system designs and the like. As data processing systems become more complex and include larger numbers of elements operable in more diverse ways, the need for comprehensive diagnostic tools able to perform diagnostic operations upon the systems in a wide variety of states and configurations becomes more important.
Another development within data processing systems aimed at reducing power consumption, so as, for example, to improve battery life, is the use of dynamic switching between operational and powered down modes. As an example, within an integrated circuit different portions, such as different processor cores, may be selectively powered up and powered down as the demand for the processing operations they can perform varies with the user's requirements.
In order for diagnostic operations to be as accurate as possible it is important that the system behavior they are attempting to investigate should be perturbed as little as possible by the investigation itself. In this context, when seeking to obtain diagnostic information concerning power-up and power-down behavior it is desirable that the circuits concerned should actually power-up and power-down rather than this merely being simulated. However, a problem arises in that diagnostic circuitry embedded within the data processing circuits being powered up and powered down will lose its configuration and so be unable to continue the diagnostic operation after the system is powered down. Continuously powering the diagnostic elements within the circuit portion being powered down is undesirable since it requires a non-contiguous power domain.